Ceramics honeycomb structures widely used for exhaust gas purification catalyst carriers are required to have a higher purification performance for coping with regulations on exhaust gas which have been yearly tightened and are further required to be reduced in pressure loss for reduction of fuel cost and increase of output.
Under the circumstances, there are increasing trends to increase the opening ratio at a cell opening end face of the honeycomb structures to reduce the pressure loss by thinning the thickness of partition walls of the honeycomb structures and to improve purification performance by reducing heat capacity of partition walls to cause early activation of catalyst after starting of engine.
On the other hand, with thinning of the partition walls of honeycomb structures, there is a new problem of occurrence of such an erosion phenomenon that various foreign matters incorporated in exhaust gas strike against the partition walls positioned at a cell opening end part to damage the partition walls.
For this problem, there has already been proposed a honeycomb structure in which the partition walls positioned at the cell opening end part have a partition wall reinforced portion (reinforced partition wall portion) increased in strength than other partition wall portions (for example, see patent document 1), and furthermore various investigations have been made on the method of providing the reinforced partition portion.
Hitherto, as a method of providing the partition wall reinforced portion, it has been known that a substrate having a honeycomb structure mainly composed of a raw material convertible into cordierite is fired, then a slurry prepared by dispersing the a raw material convertible into cordierite in a dispersion medium is applied to partition walls positioned at a cell opening end part of the substrate, and thereafter the substrate is dried and fired (for example, see patent document 1).
However, this method requires firing step which needs a long time at two stages, namely, firing of the substrate and firing for providing the partition wall reinforced portion, and thus has serious problems in production efficiency and production cost.
On the other hand, a method has been suggested which comprises applying a slurry prepared by dispersing a partition wall reinforcing material in a dispersion medium to partition walls positioned at the cell opening end part at a stage before firing the substrate of honeycomb structure, and then drying and firing the substrate, whereby firing of the substrate and formation of the partition wall reinforced portion are performed by one firing (for example, see patent document 1).
However, as to this method, no specific investigation is conducted on the difference in composition before and after the firing of the substrate. Especially, the substrate before firing usually contains organic binders and the like added for the purpose of improving strength of the partition walls, but no consideration has been given to the fact that most of the organic binders are water-soluble compounds such as methyl cellulose.
Therefore, if the step of formation of the partition wall reinforced portion which has conventionally been carried out after firing is carried out before firing using a slurry prepared by dispersing the partition wall reinforcing material in water, the organic binder dissolves in the slurry to cause distortion of partition walls of the resulting honeycomb structure to cause reduction of isostatic strength, and thus the honeycomb structure cannot be practically used.
Furthermore, in the case of using a slurry prepared by dispersing a partition wall reinforcing material in a dispersion medium, the partition wall reinforcing material settles or agglomerates due to its physical properties, and dispersibility of the partition wall reinforcing material is apt to be insufficient. Thus, variation or ununiformity of reinforcing degree tends to occur in the partition wall reinforced portion formed. Therefore, this production method suffers from the problems that a ceramic honeycomb structure having a uniform erosion resistance in the whole partition wall reinforced portion cannot be stably obtained, and besides burden of control for attaining uniform dispersion of the partition wall reinforcing material increases.
On the other hand, if a slurry prepared by dispersing a partition wall reinforcing material in a water-insoluble dispersion medium is used, the problem of reduction in isostatic strength caused by distortion of partition walls and others can be solved.
However, even this production method cannot solve the problems that a ceramic honeycomb structure having a uniform erosion resistance in the whole partition wall reinforced portion cannot be stably obtained, and besides burden of control for attaining uniform dispersion of the partition wall reinforcing material in the slurry increases.
Furthermore, with progress in thinning of the wall thickness of honeycomb structures as mentioned above, compressive strength (isostatic strength) of honeycomb structures at the time of canning considerably decreases and the honeycomb structures sometimes cannot be practically used.
(Patent document 1)
JP-A-2000-51710
The present invention has been made in view of the above problems in the conventional technologies, and the object is to provide a ceramics honeycomb structure which has an excellent erosion resistance in partition walls positioned at a cell opening end part and a high compressive strength (isostatic strength) at the time of canning and is suitable particularly as an automobile exhaust gas purification catalyst carrier, and a method for producing the ceramics honeycomb structure.